1. Field of the Invention
The present invention generally relates to thin film photovoltaic devices. More particularly, it relates to copper indium gallium diselenide/disulfide (CIGS)-based thin film photovoltaic devices.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Photovoltaic cells offer one of the most promising alternatives to energy generated from fossil fuels. To achieve commercial viability, photovoltaic cells must be fabricated inexpensively using low-cost materials and exhibit moderate-to-high efficiency in the conversion of sunlight to electricity. Additionally, for a device-manufacturing method to succeed commercially, the methods used to synthesize the material must be scalable.
The most common material used in modern photovoltaic devices is silicon. However, silicon is inflexible, expensive, and a relatively poor absorber of light. Therefore, many thin film bulk semiconductor materials have been proposed as potential replacements for silicon, for example, Cadmium Telluride, Copper Indium Gallium Sulfide (Selenide), and Amorphous Silicon. Among these, Copper Indium Gallium Sulfide (Selenide) (CuXInYGaZSASeB), also generically known as CIGS, is widely considered a promising substitute for silicon as a photoactive absorber in photovoltaic devices.
Currently, most techniques for manufacturing CIGS involve high vacuum techniques such as evaporation or sputtering. These techniques are not ideal for high-throughput operations, because they involve high-vacuum chambers, which are limited in size and difficult to implement in a high-throughput manner. As a result, it is difficult to manufacture CIGS films on large and/or on oddly shaped substrates using those techniques.
One promising alternative method for producing thin CIGS layers is by nanoparticle sintering. This technique involves making a solution or ink from nanoparticles of CIGS precursor materials and depositing layers of the ink on a substrate using one of a variety of film-forming techniques. The deposited layers are then annealed to remove solvents and other ink components from the films and to melt the nanoparticles together to yield a CIGS semiconductor layer.
The nanoparticle components of CIGS ink include nanoparticles of CIGS Precursors—copper, indium and/or gallium, and sulfur and/or selenium. Nanoparticles may include one or more of the components in a single nanoparticle. For example, nanoparticles include Cu, In, and Se to provide a CuInSe2 layer. Alternatively, the ink may contain several different types of nanoparticles, which combined provide the desired components. For example, the ink may contain nanoparticles of Cu and In and other nanoparticles of Cu and Se which, when combined, provide CuInSe2.
For CIGS nanoparticles to be useful as a starting material for photovoltaic thin films, they should possess a number of properties. Primarily, the nanoparticles must be small—ideally on the order of a few nanometers to a few hundred nanometers in diameter. Such small particles pack closely together, enabling them to coalesce more easily upon melting. Secondly, a narrow size distribution is favored to ensure that all of the nanoparticles melt at approximately the same temperature, thereby ensuring that the resulting thin film is homogeneous and of a high quality. Thirdly, it is preferred that the nanoparticles are capped with a volatile organic capping agent. Such capping agents are typically needed to help solubilize the nanoparticles in the organic solution used to deposit the nanoparticles on the substrate. It is preferred that the capping ligands be highly volatile so that they can be efficiently removed when the films are annealed. Finally, the melting temperature of the nanoparticles should be lower than the corresponding bulk material, allowing the use of lower processing temperatures.
There are a number of techniques currently used to prepare CIGS nanoparticles. Nanoparticles can be produced using colloidal methods, solvothermal methods, sonochemical methods and ball milling of bulk copper selenide.
The processes disclosed herein may be implemented with CIGS nanoparticles made using any method. However, colloidal methods are particularly promising. Colloidal methods typically involve high-temperature (>250° C.) syntheses to form nanoparticles capped with trioctylphosphine oxide (TOPO) or amines, such as “hot-injection” techniques. Hot-injection relies on the injection of small volumes of precursors into a large volume of solvent at elevated temperature. The high temperature causes breakdown of the precursors, initiating nucleation of the nanoparticles. The temperature of the reaction mixture is subsequently lowered to support nanoparticle growth over a certain period of time before quenching with a suitable organic solvent. Other methods of colloidal nanoparticle synthesis are disclosed in U.S. Patent Publication No. 2009/0139574 A1, and U.S. Pat. No. 8,563,348.
Once nanoparticles are prepared and isolated, they can be formulated into inks, which may be applied to substrates to form films. Such inks are typically solutions or suspension of the nanoparticles in an organic solvent, such as toluene, isophorone, propanol, etc. Surface-bound ligands on the nanoparticles facilitate dispersion/suspension of the nanoparticles in the ink and prevent the nanoparticles from agglomerating. The inks may also contain additional components, such as preservatives, flow or viscosity enhancers, and the like.
Films of CIGS inks can be formed on substrates using a variety of film-forming techniques, such as printing or spraying processes, spin coating, doctor blading, and the like. Once formed, the films are typically heated to expel the organic components of the film and to sinter the nanoparticles, providing a CIGS semiconductor layer.
It has been observed that removal of organic components from the film is important for obtaining high performance CIGS semiconductor layers. The presence of organic components in the film during the sintering process is believed to limit the growth of semiconductor grains. Smaller grains increase the number of grain boundaries within the resulting semiconductor layer and have an adverse effect on performance of the film. Thus, there is a need in the art for improved methods and systems of removing organic components of CIGS films during sintering.